Dry development in current electrostatic copying systems is divided into a two-component development system using a toner and a carrier, e.g., iron powder, and a magnetic one-component development system using a toner containing therein a magnetic substance. The development system using a one-component magnetic toner is advantageous over the two-component development system in that an automatic concentration controller as used in a developing machine according to the two-component development system is not necessary, thus making the machine compact and that contamination of a carrier is not involved, excluding the necessity of such maintenance as carrier exchange. On account of these advantages, the one-component development system has been spreading in not only low-speed small-sized copying machines or printers but middle- to high-speed (15 copies per minutes or more) copying machines or printers, and even more improvements in performance have been expected.
However, containing 30 to 70% by weight of a magnetic substance in general, the magnetic toner essentially has poorer fixing properties than the two-component type toners which usually contain no magnetic substance and therefore needs higher heat energy for fixing than in the two-component development system. In recent years, middle- to high-speed copying machines have undergone digitization and acquired various functions. This means that the energy allotted to a fixing means has been decreasing. To cope with this tendency, the demand for low temperature fixing of a magnetic toner has become more strict.
On the other hand, there has been an ever-increasing demand for downsizing of a copying machine itself, and studies have been directed to size reduction of not only a developing machine but a fixing means. In particular, omission of a cleaning mechanism from a fixing means, which would exclude the necessity of a winding type web or a disposable pad that has been conventionally used, has become a very important subject from the standpoint of resources saving.
Omission of a cleaning mechanism may be achieved chiefly by improving non-offset properties of a toner. In this connection, a magnetic toner involves a serious disadvantage as compared with two-component type toners containing no magnetic substance. That is, the magnetic substance contained in a magnetic toner causes many fine scratches on the surface of a fixing roller during long-term use, resulting in deterioration of the non-offset properties. The formation of fine scratches on a fixing roll is an essential problem associated with a magnetic toner containing a magnetic substance. It has therefore been demanded to develop a magnetic toner having improved non-offset properties.
Further, middle- to high-speed copying machines with a double side copying function have prevailed from the need of resources saving. Double side copying is generally carried out by once making a desired number of single-sided copies, collecting the copies on a tray in the machine, and forming an image on the reverse of the single-sided copies. In order to feed the single-sided copies one by one in an accurate manner from the tray, the rest of the single-sided copies must be held under very strong pressure. It follows that the toner image of a copy fed or the next copy which should remain on the tray received strong rubbing to cause an image smudge. This phenomenon becomes conspicuous with an increase in copying speed. Since the image made of a magnetic toner contains a magnetic substance not only in the inside of toner particles but in the vicinity of the surface thereof, it is less resistant against rubbing than the image made of toner particles containing no magnetic substance. Accordingly, the magnetic toner applied to copying machines with a double side copying function should have satisfactory resistance to rubbing (hereinafter referred to as smudge resistance).
With respect to an electrophotographic photoreceptor used in copying machines or printers, organic photoreceptors (hereinafter abbreviated as OPC) are now prevalent, which appears to be favorable for safety. However, OPC has a resin surface, it readily wears and is difficult to apply to high-speed machines. In using a magnetic toner, in particular, the very hard magnetic substance exerts an abrasive action on OPC to accelerate wear of the surface of OPC. From this point of view, too, a magnetic toner is deemed obviously inferior to two-component type toners containing no magnetic substance.
In order to improve the fixing ability of toners, various proposals have been made to date. For example, JP-B-52-3304 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a toner containing a colorant, a styrene resin and a low molecular weight polypropylene for the two-component development system. Incorporation of the low molecular weight polypropylene produces an improving effect on fixing but is still insufficient for meeting the latest strict demands. In particular, the proposal fails to satisfy the even more strict demands for a magnetic toner such that the toner should be applicable to a fixing means having substantially no cleaning mechanism, the toner should not cause smudges in double side copying, and the toner should not make scratches on a photoreceptor susceptible to abrasion, such as OPC.
In an attempt to improve theses performance properties of a magnetic toner, the amount of polyolefin such as polypropylene to be incorporated might be increased. However, while a magnetic toner in a magnetic one-component development system must be charged through friction with a toner-carrying member and be electrically attracted to the toner-carrying member to form a thin film, a polyolefin when added in an increased amount forms a thin film (called filming) on the surface of the toner-carrying member, which seriously impairs developing performance. Such a filming phenomenon of a polyolefin is attributed to poor dispersibility of a polyolefin in the toner. That is, since a polyolefin is non-polar, high crystalline, and thereby incompatible with the binder resin of the toner, it apt to agglomerate to be localized in the individual toner particles or to form independent polyolefin particles.
Besides, a polyolefin having such poor dispersibility causes broadening of the distribution of the toner charge quantity and narrowing of fog latitude, which often develops problems on practical use.
Hence, if a large amount of a polyolefin is used for the purpose of improving fixing, non-offset properties, and smudge resistance, it is very difficult to avoid the above-mentioned problems of developing properties with a one-component type toner.
To settle down these problems, it has been proposed to incorporate a polyolefin into a toner in combination with an oxidized wax or a polar group-containing wax as disclosed, e.g., JP-A-59-177570 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). According to the proposal, it appears that the polyolefin is made compatible with a binder resin of a toner owing to an oxidized wax to exhibit increased dispersibility thereby bringing some improvements. However, even if a considerably large quantity of an oxidized wax be added, the effects are not sufficient for eliminating the above-described problems, probably and partly because the polyolefin is not dispersed to a sufficient degree as necessary.
In addition, the amount of the oxidized wax to be added is limited because it adversely affects developing properties while making almost no contribution to fixing. Therefore, the combined use of an oxidized wax does not afford the means of solving the problems accompanied by the addition of an increased amount of a polyolefin.
Thus, a magnetic toner free from all the above-mentioned disadvantages has not yet been developed.